1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to an LCD device having an improved contrast ratio and driven by a low voltage.
2. Discussion of the Related Art
Recently, on entering an information society, display technologies for dealing with and displaying intensive information have rapidly developed. The LCD device has replaced a cathode ray tube (CRT) due to its low power consumption, thin profile, light weight and portability.
In general, the LCD device uses the optical anisotropy and polarization properties of liquid crystal molecules to produce an image. The liquid crystal molecules have long thin shapes that can be aligned along specific directions. The alignment direction of the liquid crystal molecules can be controlled by applying an electric field.
Accordingly, the alignment of the liquid crystal molecules changes in accordance with the direction of the applied electric field and the light is refracted along the alignment direction of the liquid crystal molecules due to the optical anisotropy, thereby displaying images.
Since the LCD device including a thin film transistor (TFT) as a switching element, referred to as an active matrix LCD (AM-LCD) device, has excellent characteristics of high resolution and displaying moving images, the AM-LCD device has been widely used. Particularly, an in-plane switching (IPS) mode LCD device using a horizontal electric field has been developed due to a wide viewing angle.
FIG. 1 is a cross-sectional view of the related art IPS mode LCD device. As shown in FIG. 1, a first substrate 1 faces a second substrate 2, and a liquid crystal layer 3 is interposed between the first and second substrates 1 and 2. First and second polarizers 4 and 6 are disposed at outer surfaces of the first and second substrates 1 and 2, respectively. The first and second polarizers 4 and 6 have optical axes perpendicular to each other. The liquid crystal layer 3 includes liquid crystal molecules 5. In addition, a plurality of pixel electrodes 7 and a plurality of common electrodes 9 are disposed on the first substrate 1. The pixel electrodes 7 and the common electrodes 9 are spaced apart from each other and alternately arranged with each other. When voltages are applied to the pixel and common electrodes 7 and 9, a horizontal electric field is generated between the pixel and common electrodes 7 and 9.
With an off state, an electric field is not generated between the pixel and common electrodes 7 and 9. The liquid crystal molecules 5 maintain their initial arrangements, and the IPS mode LCD device produces a black color.
On the other hand, with an on state, a horizontal electric field is generated between the pixel and common electrodes 7 and 9. The liquid crystal molecules 5 are arranged along a direction of the horizontal electric field, and the IPS mode LCD device produces a white color.
The IPS mode LCD device driven by the horizontal electric field has an advantage in the viewing angle.
In the IPS mode LCD device of the related art, an achiral nematic liquid crystal layer is interposed between lower and upper substrates. First and second horizontal alignment layers are formed at inner surfaces of the lower and upper substrates, respectively, to thereby orient achiral nematic liquid crystal molecules of the achiral nematic liquid crystal layer. The first and second horizontal alignment layers have an alignment direction parallel to a polarization axis of a polarizer disposed at one of outer surfaces of the lower and upper substrates, for example, disposed at the outer surface of the lower substrate.
Transmittance in the IPS mode LCD device can be expressed by equation (1) as follows:
                              T          =                                                    sin                2                            ⁡                              (                                  2                  ⁢                  α                                )                                      ⁢                                          sin                2                            ⁡                              (                                                      2                    ⁢                    π                    ⁢                                                                                  ⁢                    d                    ⁢                                                                                  ⁢                    Δ                    ⁢                                                                                  ⁢                    n                                    λ                                )                                                    ,                            equation        ⁢                                  ⁢                  (          1          )                    
where T is the transmittance, α is an angle between the director of the liquid crystal and the polarization axis of the polarizer, Δn is the refractive index, and d is a thickness of the liquid crystal layer.
From equation (1), in a black state (T=0), one of the conditions α=0, Δn=0 and d=0 should be satisfied. However, it is substantially impossible that Δn and d become zero. Accordingly, to obtain the black state, it is required to minimize a twisting of the director of the liquid crystal with respect to the polarization axis of the polarizer, that is, to meet the condition α=0.
By the way, the liquid crystal molecules, specially, nematic liquid crystal molecules are not completely aligned along the same direction due to thermal fluctuation, etc., and the nematic liquid crystal molecules are less unstable than a crystal in the degree of order. The degree of order is defined as an order parameter S expressed by equation (2). Nematic liquid crystal, generally, has an order parameter of about 0.3 to about 0.8. Here, S=1 means complete order, and S=0 means total disorder.
                    S        =                              ∫            volumeaverage                    ⁢                                    (                                                                    3                    2                                    ⁢                                      cos                    2                                    ⁢                  α                                -                                  1                  2                                            )                        ⁢                          ⅆ              v                                                          equation        ⁢                                  ⁢                  (          2          )                    
Referring to FIG. 2, which is a schematic plan view of showing relation between the director of the liquid crystal and the polarization axis of the polarizer in the related art IPS mode LCD device, a liquid crystal molecule L1 is arranged along a rubbing direction R parallel to the polarization axis P1, and α is zero, α=0. On the other hand, other liquid crystal molecules L2 and L3 are arranged slant with respect to the polarization axis P1, and α is not zero, α≠0. Thus, in total, α is not zero. Here, P2 is another polarization axis perpendicular to the polarization axis P1.
Therefore, in the IPS mode LCD device of the related art, there may be light leakage due to the disordered alignment of the liquid crystal molecules deviating from the polarization axis of the polarizer when the black state is produced. This decreases the contrast ratio of the IPS mode LCD device.